In the shifting of a stepped ratio transmission, clutches are engaged and disengaged to allow for power transfer through a plurality of different power paths. Typically, when a shift is performed, one clutch is disengaged (also known as an off-going clutch) by decreasing an oil pressure on a piston of the clutch and another clutch is engaged (also known as an oncoming clutch) by increasing a pressure on a piston of the clutch. During an overlap shift, this process happens simultaneously in a coordinated manner. In a filling phase of a shift, the piston of the ongoing clutch is positioned adjacent a plurality of friction plates by regulating a pressure of the transmission fluid.
One of the problems with filling the ongoing clutch is a repeatability of the filling process. For a system that is actuated using feedforward control, a changing system is problematic. Feedforward control means that the system responds to a control signal in a predefined way, and does not take into account a reaction based on a load. A needed width (also known as a length in time) of a pressure profile that is used to actuate a piston depends on an amount of air that is present in a plurality of hydraulic lines associated with the piston and a total length of the hydraulic lines. There is also considerable variability in an amount of oil which is present in the hydraulic lines and in the clutch. This is a result of temperature, rotational speed, a varying amount of time between shifts, and pressure dependent draining and leakage. Furthermore, some mechanical parameters of the system are uncertain. One such parameter is a stiffness of a return spring, which has a large tolerance in production. While some of these effects can be counteracted using a calibration procedure, the system is hard to accurately characterize and the system will still exhibit inconsistent behavior. Consequently, a shift quality of the system is affected negatively.
A current state of the art of control techniques does not account for a draining of the clutch. A set of optimized parameters for a filling phase is determined during a calibration session in which the clutch is repeatedly opened and closed. The process is performed with a fixed time between the opening and closing, ignoring the effect that the time between shifts has on a behavior of the system. While this method is also performed at a relatively fixed temperature, a correction factor is used during the filling process to account for the temperature.
In conclusion, the state of the art disregards for the effect of draining, aside from a temperature dependent correction on the filling time, instead of a time dependent correction. Shifts are performed using feedforward control with a considerably changing reaction. As a result, poor shifts occur in situations where the conditions vary from the parameters present during the calibration. However, even when the calibration parameters are present, large variability can have a detrimental effect on the shift quality. FIG. 1 illustrates several consecutive fillings that were performed with the same or similar pressure signals, and are shown using dashed lines. As shown in FIG. 1, a plurality of measured response to the pressure signals, shown using solid lines, differ vastly.
The measured response is dependent on a temperature as well as a time between shifts. A precise correction for both temperature and time between shifts is needed. The state of the art only contemplates temperature compensation. While a compensation for the time between shifts could also be added, the number of parameters influencing the system makes such a task increasingly complex. Furthermore, it is expected that large variability would still remain, despite compensating for the time between shifts.
It would be advantageous to develop a method of prefilling a hydraulic clutch that increases a repeatability of a clutch filling process, accounts for a draining of the clutch, accounts for a temperature at which a shift is performed, and accounts for a time between shifts.